A leucine-inserting tRNA has been transformed into a serine-inserting tRNA bychanging 12 nucleotides. Only 8 of the 12 changes are required to effect the conversion ofthe leucine tRNA to serine tRNA identity. The 8 essential changes reside in basepair 11-24in the D stem, basepairs 3-70, 2-71 and nucleotides 72 and 73, all of the acceptor stem.
Functional amber suppressor tRNA genes were generated for 14 species of tRNA inE. coli, and their amino acid specificities determined. The suppressors can be classifiedinto three groups, based upon their specificities. Class I suppressors, tRNA^(Ala2)_(CUA),tRNA^(GlyU)_(CUA), tRNA^(HisA)_(CUA), tRNA^(Lys)_(CUA), and tRNA^(ProH)_(CUA), inserted the predictedamino acid. The Class II suppressors, tRNA^(GluA)_(CUA) , tRNA^(GlyT)_(CUA), and tRNA^(Ile1)_(CUA)were either partially or predominantly mischarged by the glutamine aminoacyl tRNAsynthetase (AAS). The Class III suppressors, tRNA^(Arg)_(CUA), tRNA^(AspM)_(CUA), tRNA^(Ile2)_(CUA),tRNA^(Thr2)_(CUA), tRNA^(Met(m))_(CUA) and tRNA^(Val)_(CUA) inserted predominantly lysine.
